The term “wire” as used herein refers to a conductive core, wherein the conductive core is enveloped by at least one insulative layer. The term “wire” as used herein also encompasses cables, or groups of two or more insulated conductive cores.
Wires have been ubiquitous since at least the Industrial Age for all types of electrical applications. These applications include, without limitation, commercial and residential power supply, appliances, computers and personal electronics of all shapes and sizes, vehicles of all types, including fossil fuel-powered and electrically-powered automobiles and recreational vehicles.
Historically, wires were manufactured by a simple heat-curing method. The historical heat-curing method involved feeding a conductive core into an extruder wherein at least one insulative layer was extruded about the conductive core. To form insulative layers using such methods, all starting materials, including cross-linkable polymers and their associated curing agents, were combined in an extruder prior to extrusion. Then, the starting materials were extruded about the conductive core at temperatures ranging from about 80° C. to about 110° C. depending upon the particular materials. Next, the extruded wire pre-product was heat cured at temperatures ranging from about 135° C. to about 155° C. for a length of time to cause sufficient cross-linking in the insulative layer or layers to confer onto the insulative layer or layers the desired properties, including physical, mechanical and/or electrical properties.
Such historical heat-curing methods were efficient and relatively inexpensive. For example, by adding all of the starting materials to the extruder at roughly the same time, manufacturers may have realized a gain in manufacturing efficiency. That is, manufacturers could avoid slowing manufacturing line speeds and could avoid purchasing additional equipment to manage the addition of separate materials at separate times.
However, historical heat-curing methods faced numerous challenges. For example, manufacturers sought to avoid premature cross-linking during extrusion, also known as scorching. Significant scorching could damage extrusion equipment and generate wire that would not meet technical specifications, including physical, mechanical and/or electrical specifications. Accordingly, manufacturers were left to experiment with polymer and curing agent combinations to minimize scorching.
Eventually, technical demands on wire became more sophisticated, and wire produced by historical heat-curing methods failed to satisfy a variety of technical specifications. This occurred in many industries. By way of non-limiting example, in the automotive industry, certain original equipment manufacturers (OEMs) require wire to withstand scrape abrasion such that when a conductive core of a wire has a cross-sectional area of 0.22 mm2 or less, the insulation on the wire remains intact following 150 cycles of abrasion scrapes with a needle having a diameter of 0.45±0.01 mm. Wire manufactured by historical heat-curing methods does not satisfy this standard.
To meet the growing technical demands on wire, manufacturers increasingly turned away from historical heat-curing methods and toward radiation or electron beam (e-beam) manufacturing methods. Indeed, e-beam manufacturing methods remain in use today.
E-beam manufacturing methods typically involve feeding a conductive core into an extruder where at least one insulative layer is extruded about the conductive core. To form an insulative layer, all starting materials for the layer are added to the extruder. Then, the starting materials are extruded about the conductive core. Next, the extruded wire pre-product is collected on a spool before being exposed to radiation. Radiation initiates curing, so curing agents are not typically used in e-beam manufacturing methods.
E-beam manufacturing methods have advantages over historical heat-curing methods. As non-limiting examples, the cross-linking reaction in e-beam manufacturing methods is faster and more uniform, especially for thin wall wires. The e-beam manufacturing methods produce wire that satisfies more challenging technical specifications. As a non-limiting example, e-beam manufacturing methods are more effective at preparing abrasion-resistant wires and ultra thin wall wires with a temperature class rating of Class D (150° C.) or higher.
E-beam manufacturing methods, however, also involve numerous challenges. The equipment is expensive and there are attendant safety procedures and precautions whenever radiation is used in a manufacturing method. These safety efforts can add to expenses and slow manufacturing line speeds. Additionally, e-beam manufacturing methods may be more difficult to use with thick wall wires. This may be because, at commercially acceptable manufacturing line speeds, there is a potential for incomplete penetration of electron beams through a dense polymeric insulative layer or layers. Incomplete penetration can lead to incomplete curing, which in turn can cause wire to fail technical specifications. For example, the insulation of the wires may swell or crack.
Additionally, using e-beam manufacturing methods to form very flexible wire presents challenges. This may be because, to spool extruded wire that is not yet cured (that is, extruded wire pre-product), the insulative layer or layers must be sufficiently hard to avoid becoming misshapen or deformed. Generally, this requires the extruded wire pre-product to have a hardness of about 80 Shore A or higher. After curing, the cross-linked polymer in the wire causes the wire to be substantially harder than the extruded wire pre-product. As a result, wire made by e-beam manufacturing methods can fail to achieve flexibility-related mechanical properties desired for certain industrial applications. By way of non-limiting example, it may be useful to produce a flexible wire having a tensile stress at yield of less than 9 MPa and a tensile modulus at 200 MPa. Wire produced by e-beam manufacturing methods would not be expected to exhibit such mechanical properties.
Accordingly, there is a need for improved manufacturing methods and wires. Efficient and cost effective methods are desired that can produce wires that meet can meet increasingly demanding technical specifications.